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Langmuir 1998, 14, 3052-3061
Ordered Adlayers of a Nonplanar Molecule on a Surface: Misfit Monolayers and Intercalated Bilayers as the Result of a Dialkyl Amino Group Christopher B. Gorman,* Igor Touzov, and Russell Miller Department of Chemistry, Box 8204, North Carolina State University, Raleigh, North Carolina 27695 Received July 17, 1997. In Final Form: February 27, 1998 Monolayer and bilayer structures of a nonplanar molecule, 5-(N,N-didecylamino)-2,4-pentadienal, physisorbed onto highly oriented pyrolytic graphite have been characterized using several scanning probe techniques, including scanning tunneling microscopy, and both contact and noncontact atomic force microscopy. These structures indicate several important components of this molecule in determining and enforcing an ordered packing motif that may be generalizable to other supramolecular design schemes. In the monolayer structure, the geometry of the amino group enforces a motif in which part of the molecule lies upon the graphite surface and one of the alkyl chains extends away from the surface. This extended chain is a structure director for the formation of bilayer structures. The apparent thickness and stability of the bilayer structures are shown to be dependent on the presence of water or other admixtures, suggesting intercalation of small, polar molecules into this bilayer.
Introduction The advent and proliferation in the use of scanning probe microscopes has resulted in the study of a number of molecular adlayers on atomically flat surfaces.1,2 Initially, it was of interest to illustrate various adlayer packing motifs and to make structural assignments based on the periodicities observed in molecular-scale images. With such information in hand, it is increasingly pertinent to ask what range of molecular shapes can pack into wellordered, characterizable monolayers (ML) on a surface, whether other molecular features can be incorporated that stimulate organization in more than one spatial direction, and how functional elements might be incorporated into such molecular adlayers without sacrificing a high degree of intermolecular order. Such functionalities include elements for molecular recognition,3-7 electroactive moieties,8-13 and conjugated molecules.14 It has been shown that planar, conjugated molecules such as oligothiophenes15-19 and phenyl(1) Gu¨ntherodt, H.-J.; Wiesendanger, R., Ed. Scanning Tunneling Microscopy I. General Principles and Applications to Clean and Adsorbate-Covered Surfaces; Springer-Verlag: New York, 1992; Vol. 20. (2) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM; VCH Publishers: New York, 1996. (3) Moy, V. T.; Florin, E.-L.; Gaub, H. E. Science 1994, 266, 257-259. (4) Musil, C. R.; Jeggle, D.; Lehmann, H. W.; Scandella, L.; Gobrecht, J.; Do¨beli, M. J. Vac. Sci. Technol. B 1995, 13, 2781-2786. (5) Oishi, Y.; Torii, Y.; Kuramori, M.; Suehiro, K.; Ariga, K.; Taguchi, K.; Kamino, A.; Kunitake, T. Chem. Lett. 1996, 411-412. (6) Steinbeck, M.; Ringsdorf, H. Chem. Commun. 1996, 1193-1194. (7) Takami, T.; Delamarche, E.; Michel, B.; Gerber, C.; Wolf, H.; Ringsdorf, H. Langmuir 1995, 11, 3876-3881. (8) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. J. Phys. Chem. 1996, 100, 11207-11210. (9) Lu, X.; Hipps, K. W.; Wang, X. D.; Mazur, U. J. Am. Chem. Soc. 1996, 118, 7197-7202. (10) Bard, A. J.; Fan, F.-R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou, F. Science 1991, 254, 68-74. (11) Fan, F.-R. F.; Bard, A. J. Science 1995, 267, 871-874. (12) Fan, F.-R. F.; Kwak, J.; Bard, A. J. J. Am. Chem. Soc. 1996, 118, 9669-9675. (13) Snyder, S. R.; White, H. S. J. Electroanal. Chem. 1995, 394, 177-185. (14) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319-3320.
acetylene oligomers14 can form ordered structures on a surface. Such structures can lie with their principal axis both parallel and roughly perpendicular20,21 to the surface depending on the molecular structure, end group functionality and surface employed. However, all of the structures of this type, have basically been restricted to molecules that can exist entirely in one plane. Most typically, these structures are linear as well. Many potentially useful molecular functionalities do not neatly fit into this structure type, however. For example, a dialkyl amino group has local trigonal symmetry. This element completely prevents a linear packing motif and frustrates two-dimensional packing as well.22 In this paper, it is shown that a dialkyl amino functionalized molecule, 5-(N,N-didecylamino)-2,4-pentadienal (DAPDA), can form an ordered adlayer on a highly oriented pyrolytic graphite (HOPG) surface that can be probed by both scanning tunneling microscopy (STM) and atomic force microscopy (AFM) and that this group acts as a structure director for molecular organization both parallel and perpendicular to the HOPG surface. It will be shown that, in this structure, the conjugated portion of the molecule and one of the alkyl chains attached to the nitrogen atom lie parallel to the surface. The second alkyl chain extends away from the surface. This second chain (15) Ba¨uerle, P.; Fischer, T.; Bidlingmeier, B.; Stabel, A.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 4. (16) Bo¨hme, O.; Ziegler, C.; Go¨pel, W. Adv. Mater. 1994, 6, 587-589. (17) Liu, T.-L.; Parakka, J. P.; Cava, M. P.; Kim, Y.-T. Langmuir 1995, 11, 4205-4208. (18) Mu¨ller, H.; Petersen, J.; Strohmaier, R.; Gompf, B.; Eisenmenger, W.; Vollmer, M. S.; Effenberger, F. Adv. Mater. 1996, 8, 733-737. (19) Soukopp, A.; Glo¨ckler, K.; Ba¨uerle, P.; Sokolowski, M.; Umbach, E. Adv. Mater. 1996, 8, 902-906. (20) Scho¨nherr, H.; Kremer, F. J. B.; Kumar, S.; Rego, J. A.; Wolf, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Am. Chem. Soc. 1996, 118, 13051-13057. (21) Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir 1996, 12, 5742-5744. (22) Double chain surfactants based on a dialkyl amino group are known. In an AFM study of a Langmuir Blodgett film of one of these, the molecules were believed to be oriented roughly perpendicular to the surface but molecular scale periodicities were not observed (Oishi, Y.; Kato, T.; Kuramori, M.; Suehiro, K.; Ariga, K.; Kamino, A.; Koyano, H.; Kunitake, T. Chem. Lett. 1996, 857-858).
S0743-7463(97)00805-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/30/1998
Ordered Adlayers of a Nonplanar Molecule
Langmuir, Vol. 14, No. 11, 1998 3053
will be shown to play a role in the formation and stability of bilayer (BL) structures of these molecules. Overall, the structures characterized here represent a new layer packing motif of functional molecules with applicability to design strategies for supramolecular structures on surfaces.
Experimental Section General Considerations. All starting compounds were purchased from Aldrich Chemical and were used without further purification unless otherwise noted. Flash chromatography was run on 230-400 mesh silica gel (Merck). Nuclear magnetic resonance characterization was performed at 300 MHz (1H) or 75 MHz (13C) on a NMR system from General Electric. High resolution mass spectrometry (HRMS) was performed at the North Carolina State mass spectrometry facility. The HOPG was provided by the Moscow Institute of Graphite. Synthesis of 5-(N,N-Didecylamino)-2,4-pentadienal. The synthesis of DAPDA was based on a modification of the procedure of Malhotra and Whiting.23 The compound was prepared from N-(2,4-dinitrophenyl)-pyridinium chloride and didecylamine. To form N-(2,4-dinitrophenyl)-pyridinium chloride, pyridine (3.91 g, 49.5 mmol) was added to a solution of 2,4-dinitrochlorobenzene (5.00 g, 24.7 mmol) in acetone. This solution was stirred at reflux under nitrogen for 2 h. After cooling, the precipitate was filtered and washed with petroleum ether to yield 6.95 g (62%). This compound was used without further purification. A portion of crude N-(2,4-dinitrophenyl)-pyridinium chloride (200.9 mg, 0.712 mmol) in ethanol was heated to reflux with two equivalents of didecylamine (502.8 mg, 1.42 mmol) for 1 h. The red reaction mixture was poured into water (20 mL) and filtered. To the filtrate was added 5 M NaOH (2 mL). After stirring for 1 h, the mixture was extracted with chloroform. The chloroform layer was washed with water, dried over MgSO4, and evaporated to dryness. The crude product was purified by flash chromatography, eluting with 20% ethyl acetate/pet ether to yield 268.8 mg (56%). 1H NMR (CDCl3): δ 0.89 (t, 6H, J ) 6.5 Hz), 1.28 (s, 28H), 1.57 (m, 4H), 3.15 (t, 4H, J ) 7.5 Hz), 5.28 (dd, 1H, J1 ) J2 ) 12.0 Hz), 5.82 (dd, 1H, J1 ) 14.3 Hz, J2 ) 8.4 Hz), 6.75 (d, 1H, J ) 12.8 Hz), 7.09 (dd, 1H, J1 ) J2 ) 13.0 Hz), 9.27 (d, 1H, J ) 8.4 Hz); 13C NMR (CDCl3): δ 14.32, 22.89, 27,04, 29.50, 29.73, 32.09, 96.99, 119.46, 151.84, 157.28, 192.31; HRMS: calcd for C25H47NO, 378.3736; found: 378.3741, ∆ ) 1.4 ppm. Procedure for Liquid-Phase Deposition of DAPDA onto HOPG. A drop of 1 mM DAPDA solution in methanol was deposited on a piece of freshly cleaved HOPG. The solution was wicked off of the HOPG by placing a Kimwipe against one edge. To prepare a surface in which the DAPDA concentration was minimized, the HOPG was angled slightly with the lower edge in contact with a Kimwipe, allowing the deposition solution to roll quickly across the surface and then be wicked off. During imaging of this sample, sections of the surface near the top of this inclined plane during sample preparation showed less DAPDA coverage than sections of the surface near the bottom. In all cases, this modified surface was then pumped into a heliumfilled glovebox containing the STM/AFM, mounted in the microscope, and imaged without further delay. Procedure for Vapor-Phase Deposition of DAPDA onto HOPG. A 10-cm piece of 8-mm diameter standard glass tubing was sealed at one end, cleaned with hydrofluoric acid, rinsed with water (HPLC grade), and dried in an oven. The clean, dry tube was cooled under reduced pressure. Upon reaching room temperature, the vacuum was broken and a small drop of DAPDA was placed in the sealed end of the tube with a pipet, followed by a freshly split piece of HOPG that had been cut to fit. The liquid and the HOPG were not allowed to come into contact with one another and were typically ∼3 cm away from each other. The tube was evacuated and sealed. This procedure was modified to yield a total of three different deposition conditions. The condition (23) Malhotra, S. S.; Whiting, M. C. J. Chem. Soc. 1960, 3812-3822.
Figure 1. Models for two possible conformations for DAPDA when physisorbed to HOPG. just described is a “wet tube”, where the tube was not further dried. The “partially wet tube” condition was achieved by flame drying half the tube before sealing, and the “dry tube” condition was produced by quickly flame drying the entire tube before sealing. The sealed tubes were placed in an oven at 120 °C for 1-4 days. After the deposition was complete, the tube was opened in a helium-filled drybox containing the STM/AFM, mounted in the microscope, and imaged without further delay. STM and AFM Measurements. All measurements were carried out in dry atmosphere of prepurified helium with a relative humidity of
